308 ANATOMY OF THE OENTBAL NEEVOUS SYSTEM. 



1. Midbrain-roof : (a) Corpus quadrigeminum anterius, from which 

 the optic nerve arises dorsally and the deep medullary stratum ventrally; 

 (&) the decussation of the latter above the aqueduct; (c) the central gray 

 matter which surrounds the aqueduct; (d) at the outer margin of the central 

 gray matter lies a small nucleus (not before mentioned), whose vesicular cells 

 may be found at the same relative position in all sections of the midbrain. 

 Prom it arises a slender bundle of fibers, which passes down, receiving acces- 

 sions continuously to the pons, where it joins the emerging fibers of the 

 trigeminus. It is the midbrain-roof of the nervus trigeminus. {Bad. desc. 

 7, Fig. 199.) 



In the roof of the midbrain one may distinguish a rather faint stratification of 

 alternating gray and white matter. Tlie minute structure of the layers is not 

 sufficiently known in the human brain. In mammals one may generally differ- 

 entiate in the anterior quadrigeminal body the following layers from without in- 

 ward: 1. Superficial marrow and gray matter, — the entering fibers of the optic 

 tract, — atrophies after extirpation of the ej'e, and is rudimentary in the mole 

 (Ganser). 2. Middle gray matter, a direct continuation of the superficial gray 

 matter; best studied in birds, and, according to Cajal and Gehiicliten, it contains 

 numerous cells whose neuraxons usually pass Aown into the fillet, but whose den- 

 drites break up into twigs among the fine terminal ramifications which the optic 

 nerve sends into the superficial gray matter. 3. Middle marrow. It lies within and 

 below the middle gray and contains bundles from the opticus radiation, but also 

 other fibers, as it degenerates only in part after removal of the corresponding cor- 

 tex. 4. The deep gray matter and ilir deep niarroir, or deep medullary stratum. 

 The gray matter is the continuation of the general gray matter of the quadrigeminal 

 bodies. The deep marrow contains the deep fibers of the stratum lemnisci, which 

 spring from the deep and middle gray of the quadrigeminal body. 



2. The Tegmentum : {a) In the ventral portion of the central gray mat- 

 ter, the nucleus posterior medialis et lateralis of the nervus oculo-motorius, in 

 which pass fibers apparently from the deep marrow and some from the pos- 

 terior longititdinal fascicle. (&) Lateral from and bordering on the posterior 

 longitudinal fascicle, the fibers of the posterior commisstire. (c) External 

 to (b) a meduUated area which comes from the thalamus and which left it as 

 laminae medullares; it probably contains a bundle from the nucleus of the 

 trigeminus to the thalamus, (d) The loiver fillet from the quadrigeminal 

 bodies, and the upper fillet from the thalamus, (e) The nucleus ruber teg- 

 menli, from which arise numerous fibers for the superior cerebellar 

 peduncles. (/) The "fountain-lilce" tegmental decussation, (g) The fas- 

 ciculus retroflexus. 



3. On the boundary between the crusta and the tegmentum one recog- 

 nizes the substantia nigra Sommeringi, into which numerous fibers (the 

 stratum intermedium) pass from the nucleus lentiformis. 



4. The Pes Pedunculi: (a) The pyramidal tract is shown still non- 

 meduUated, as it appeared in a specimen from a child four weeks of age. 



